The ability to reproduce realistic arterial flow waveforms in vitro is essential in the study of vascular haemodynamics. Simulated pulsatile flow has been used extensively in previous investigation of flow in arterial models with bifurcations and stenoses. Many different techniques have been used to measure flow in these models, including (1) laser Doppler anemometry (Ku, D. N, and Giddens, D. P. (1987): Laser Doppler Anemometer Measurements of Pulsatile Flow in a Model Carotid Bifurcation J. Biomechanics, 20, 407-421), (2) Doppler ultrasound (Cho, Y. I., Back, L. H., Crawford, D. W., Cuffel, R. F. (1983): Experimental Study of Pulsatile and Steady Flow Through a Smooth Tube and an Athersclerotic Coronary Artery Casting of Man, J. Biomechanics, 16, 933-946.); and (Fei D. Y., Billian, C., Rittgers, S. E. (1988): Flow Dynamics in a Stenosed Carotid Bifurcation Model-Part 1: Basic Velocity Measurements, Ultrasound in Med. & Biol., 14, 21-31), (3) magnetic resonance (Evans, A. J., Hedlund, L. W., Herfkens, R. J., Utz, J. A., Fram, E. K., (1987): Evaluation of Steady and Pulsatile Flow with Dynamic MRI Using Limited Flip Angles and Gradient Refocused Echoes, Magnetic Resonance Imaging, 5, 475-482); and (4) digital radiography (Cunningham, I. A., Yamada, S., Hobbs, B. B., Fenster A., (1989): Arterial Flow Characterization with a Photodiode Array Based Imaging System. Med. Phys., 16, 179-187).
Physiological pulsatile flow waveforms are also required to investigate the role of pulsatility in tissue perfusion (Tranmer, B. I., Gross, C. E., Kindt, G. W., Adey, G. R., (1986): Pulsatile Versus Nonpulatile Cardiovascular Studies, Med. & Biol. Eng. & Comput., 22, 86-89).
Finally, the ability to mimic arterial flow is essential for quality assurance and calibration of all clinical techniques of blood flow measurement, such as Doppler ultrasound (McDicken, W. N. (1986): A Versatile Test-object for the Calibration of Ultrasonic Doppler Flow Instruments, Ultrasound in Med. & Biol., 26, 245-249); and Shortland, A. P., Cochrane, T., (1989): Doppler Spectral Waveform Generation in vitro: An Aid to Diagnosis of Vascular Disease, Ultrasound in Med. & Biol., 15, 737-748).
Most of the prior art techniques for the investigation of time-varying flow require gated acquisition of many cardiac cycles, so cycle-to-cycle variability in the flow waveform must be small. Longterm stability is equally important for quality assurance applications where the flow source may be used for absolute calibration of clinical instruments. Therefore, a blood flow simulator must be capable of producing a wide range of flow rates in order to simulate flow in the peripheral vasculature, where peak flow rates of 30 ml s.sup.-1 have been reported (Marquis, C., Meister, J.-J., Mooser, E., and Misoman, R., (1986): Quantitative Pulsed Doppler Measurement of Common Femoral Artery Blood Flow Variable during Postocclusive Reactive Hyperemia, J. Clin. Ultrasound, 14, 165-170). It must be easily programmed to produce a variety of pulsatile waveforms, including waveforms with flow reversal. The simulator must be capable of producing continuous steady flow, which is required as the basis for many experimental investigations and calibration procedures. It is essential that a pumping mechanism not produce gas bubbles or cavitation, since bubbles change the hydrodynamic properties of the fluid, and their presence will produce measurement artifacts, particularly with ultrasound instrumentation. Finally, a device to stimulate physiological flow should operate as an ideal flow source, capable of generating sufficient pressure to be unaffected by changes in the peripheral resistance of the model vascular system under investigation.
Many different pumps have been proposed to meet these requirements, and Law, Y. F., Cobbold, R. S. C., Johnson, K. W., Bascom, P. A. J., (1987): Computer-controlled Pulsatile Pump System for Physiological Flow Simulation. Med. & Biol. Eng. & Comput., 25, 590-595 provides a thorough review of previous work. Briefly, prior art devices can be categorised according to their basic pump type; gear, peristaltic or piston.
Gear pumps have been used (Issartier, P., Sioffi, M., Pelissier, R., (1978): Simulation of Blood Flow by a Hydrodynamic Generator, Med. Prog. Technol., 6, 39-40) to generate pulsatile waveforms. However, the drawbacks of this approach include damage to suspended particles and sensitivity to cavitation due to the action of the gears.
Modified peristaltic pumps have been used (Douville, Y. Johnston, K. W., Kassam, M., Zuech, P., Cobbold, R. S. C., Jares, A., (1983): An in vitro Model and its Application for the Study of Carotid Doppler Spectral Broadening, Ultrasound in Med. & Biol., 14, 21-31; Law, Y. F., Cobbold, R. S. C., Johnson, K. W., Bascom, P. A. J., (1987): Computer-Controlled Pulsatile Pump System for Physiological Flow Simulation, Med. & Biol. Eng. & Comput., 25, 590-595) to simulate physiological flow waveforms by mechanical manipulation of the backplate or computer-control of the roller. This approach allows the production of only a limited subset of waveforms and is not well suited to the production of steady flow. It is also difficult to program new waveforms, or produce reverse flow with this technique.
Cam-driven piston pumps have been used (Kiyose T. A., Kusaba, M., Inokuchi, Y., Takamatsu, U., (1977): Development of a Pump System for Experimental Model Simulation of Blood Flow in Peripheral Artery, Fucuota Acta Med., 68, 86-91; Appugliese R., Jares, A., Kassam, M., Johnston K. W., Cobbold, R. S. C., Hummel, R. L., Arato, P. (1980): Pulsatile Visualization for in-vitro Haemodynamic Studies Related to Doppler Ultrasound, Dig. 8th Can. Biol. Eng.. conf., 3-4; and Poots, K., Cobbold, R. S. C., Johnston, K. W., Appugliese, R., Kassam, M., Zuech, P. E., Hummel, R. L., (1986): A New Pulsatile Flow Visualization Method Using a Photochromic Dye Application to Doppler Ultrasound, Ann. Biodmed. Eng., 14, 203-218) to simulate peripheral arterial flow. This class of pumps share the general disadvantages of difficulty in programming new waveforms and difficulty in producing steady flow.
All of the flow simulators described above have one other significant disadvantage, namely that some form of flow monitoring must be performed to provide feedback and to determine the output waveform.
A variation of flow simulator reported by Werneck, N. M., Jones, N. B., Morgon, J., (1984): Flexible Hydraulic Simulator for Cardiovascular Studies, Med & Biol. Eng. & Comput., 22, 86-89, makes use of a servomotor driven piston pump which acts as an ideal flow source. This approach overcomes many of the limitations of previous designs, but is not well suited to the production of uninterrupted constant flow.